FIG. 1 shows circuitry 400 of a single-switch flyback converter in examples of the present disclosure. The single-switch flyback converter includes a switch 420, a transformer 440 and a resistor 460. The transformer 440 has a primary winding 442 and a secondary winding 444. A first end of the switch 420 is connected to a first end of the primary winding 442 of the transformer 440. A second end of the switch 420 is connected to the first end of the resistor 460. A second end of the resistor 460 is connected to a ground.
FIG. 2 shows a printed circuit board (PCB) layout 500 for a conventional single-switch flyback converter. The PCB layout 500 is configured to receive a conventional DMOS device. The conventional DMOS device has a small area source lead attached to a small copper pad 510 on the PCB and a large area drain lead 540 attached to a large copper pad area 520 on the PCB. The DMOS chip is above the large copper pad area 520 overlapping the large area drain lead 540. The drain electrode of the DMOS chip is connected to the transformer TX1 through the large area drain lead 540 and large copper pad area 520. The source electrode of the DMOS chip is connected to ground through resistor R2. The performance of the PCB layout 500 is not optimized due to necessary tradeoff between thermal dissipation and EMI noise reduction. The DMOS device Q1 is hot and needs a large copper pad area 520 (for example, larger than 10 mm in length and 5 mm in width) for cooling. However, the large area drain lead 540 has high voltage and has high dv/dt value. It couples electromagnetic interference (EMI) noise to the system. This may not be a problem for low voltage application. However for high voltage applications such as 500V or higher, the EMI noise is high due to the fast changing and high drain voltage. It requires a small copper pad area 520 to reduce the EMI noise. This is in contrary to larger copper pad area 520 for cooling purpose. The tradeoff of a large copper pad area 520 is large EMI noise. Furthermore, for high voltage applications, the high voltage drain lead with large area will demand large safety space therefore increasing the device area, making it challenge to minimize the device size while keeping safety space for high voltage.
It is advantageous to implement an improved DMOS package in high voltage flyback application to reduce the EMI. It is advantageous to improve the DMOS package by reducing the drain lead area to reduce the EMI of the flyback convertor, and further to add an insulation material between the DMOS chip and a lead frame, to introduce V-shaped grooves in the lead frame, and to use an island-type lead frame (with raised portions) because it results in better thermal performance by having a large copper pad area yet still with less electromagnetic noise.